Replace hoverxref with RTD-native links preview

[jakkdl]

We're currently using https://pypi.org/project/sphinx-hoverxref/ but it's been deprecated and incorporated into ReadTheDocs itself as https://docs.readthedocs.com/platform/stable/link-previews.html

That also means we get rid of an ugly warning readthedocs/sphinx-hoverxref#312 (comment)